Teenage climate activist Greta Thunberg is on her way to New York this week for the U.N. Climate Summit, but instead of a multi-hour flight, she’ll be making a two-week, low-carbon trip—by sailing yacht. Meanwhile, you may be asking yourself: is that really necessary?
It may be an extreme option, but to really rein in your carbon footprint (or, in Thunberg’s case, to maintain your credibility as a climate activist), the answer is, in fact—yes.
So much so, that even climate scientists are trying to do it, too.
“I swear I had the idea before her!” says Kimberly Nicholas, an associate professor at the Lund University Centre for Sustainability Studies, in Lund, in Sweden. “She always beats me!”
Nicholas, an admirer of Thunberg’s, is only being partially tongue-in-cheek: the Californian moved to Sweden in 2010 and stopped taking flights within Europe in 2012 after “finally facing the cognitive dissonance I was having about flying to [climate change] conferences.”
Now, she and her husband are taking sailing lessons, with the aim of traversing the Atlantic in 2023, in order to visit family in the U.S.
The figures Nicholas alludes to are stark. In a paper she and collaborator Seth Wynes at the University of British Columbia published in 2017, they estimated that the total carbon impact of a single flight is so high that avoiding just one trip can be equivalent to going car-free—for a year.
“I think it’s important for people to know that every flight you can avoid [is] a major climate move,” says Nicholas.
Fly once—or drive for a year
Flying is nearly always the most carbon-intensive way to travel, relative to cars, buses, or trains. Exact comparisons to other forms of transport vary, but the difference—particularly compared to rail travel—tends to be one of magnitude.
By one European Union estimate, flying is about eight and a half times as carbon intensive as rail, the lowest-carbon form of transport, when measuring the average output per passenger, per kilometer.
Other analysis from Michael Sivak, former director of the Sustainable Worldwide Transportation unit at the University of Michigan, presented the data this way: a single economy flight from San Diego to Frankfurt, Germany is about equal in output to driving a light-duty vehicle for a year.
Flying first class? Then you’re sat in a heavier seat, taking up more space—leaving you with just a single domestic flight, from Atlanta to Los Angeles.
It’s not just long-haul flights that have a huge footprint, either. Because a large proportion of fuel is burned during take-off, a shorter flight can actually have a disproportionate carbon impact per kilometer.
Put another way: even the most devoted activist could scupper much of their carbon-reducing efforts for the rest of the year on just a single round-trip.
The jet-setter emissions
The huge impact of a single flight might seem misleading, because aviation, taken from a distance, doesn’t look like a large contributor to emissions.
Transport overall is the largest share of energy-related CO2 output, with 24% of direct emissions according to the International Energy Agency. But aviation makes up about 2% of total emissions, according to the International Air Transport Association. Meanwhile, planes have generally become more efficient every year. Because fuel is such a huge cost for airlines—even though it goes largely untaxed on international flights—operators have a powerful incentive to make a more efficient plane.
But gains in efficiency, and increasing industry efforts to set targets to curtain emissions, can’t keep pace with the powerful growth of demand: from 2000 to 2014, the sector grew by 140%, according to the IEA, and the growth of low-cost airlines has increasingly democratized air travel—even on transatlantic flights.
That makes aviation one of the most difficult sectors to decarbonize, according to the agency.
Your flight footprint
On a personal level, that means that if you fly regularly, it will probably be the largest contribution to your personal carbon footprint.
Conversely, reducing how much you fly is one of the single most significant things you can do to cut your personal impact, alongside steps like eating less or no meat, and not driving a car.
In fact, there is a growing movement against flying, particularly in Thunberg’s Sweden, where flygskam, or “flight shame” has pushed travelers, and not just climate scientists and activists, to choose lower-carbon transit options.
But that’s easier for Europeans to do, since the region is relatively well-connected through rail infrastructure. What if you have to fly over an ocean? Then your options, like Thunberg’s, are fairly limited.
A round-trip cruise from the U.K. to New York—even if you can afford it—is likely to set you back at least 12 to 16 days, with a carbon output that would surpass a London to New York flight, according to myclimate, a Switzerland-based NGO that offers carbon offsets.
By contrast, the yacht Thunberg and her father will be sailing will be zero-carbon, outfitted with solar panels and underwater turbines to supply electricity. But what they gain in energy efficiency, they may lose in comfort: among other things, their sailboat lacks a proper toilet.
More must-read stories from Fortune:
—The newest threat to cheap European flights? A plan to finally tax jet fuel
—Energy company earnings suffer in the gas glut era
—Renewable energy is booming. Here’s how to keep it going
—Why kids are skipping school to fight climate change
—Listen to our audio briefing, Fortune 500 Daily
Follow Fortune on Flipboard to stay up-to-date on the latest news and analysis.
